1. Field of the Invention
This invention relates to reproduction of photographic images, and more particularly, to electronically generating a screened image, which at any point can be either black or white, but not an intermediate gray level. Such devices include thermal transfer fax machines, laser electrostatic, and inkjet printers. The invention further relates to adjusting the coarseness of the resulting screened image.
2. Description of the Related Art
A typical electronic device for reproducing photographic images consists of a scanning module, a screening module, and a marking module. The scanning module is used to sense the gray shade of each point of the original photographic image, and report it in electronic form. The screening module processes this data into a form suitable for marking. Because many marking devices can only reproduce black or white at any given point, and not intermediate shades of gray, the screening module must generate a screened image containing only black and white points. Electronic signals representing the screened image are then routed to the marking module, which marks a medium such as paper or photographic film with the black and white points corresponding to the image generated by the screening module.
One technique used in a screening module is electronic simulation of a conventional screening technique. The conventional technique is described in U.S. Pat. No. 498,127, xe2x80x9cScreen For Making Photomechanical Printing Plates.xe2x80x9d A state-of-the art electronic simulation is described in U.S. Pat. No. 4,012,584. This technique simulates gray shades by varying the size of the dots. However, the number and position of the dots remains constant. When used with marking modules exhibiting low spatial resolution, this technique suffers two problems. First, the screen patterns are coarse. Second, the number of shades that can be distinguished is small, also causing degradation in the reproduction quality. Conventional techniques which vary the size of the dots have not employed adaptive or recursive methods.
Conventional screening techniques have been employed with repeated cells of pixels in fixed locations. Often these fixed cells are rotated to enhance the pattern to the viewer. Since fixed cells have a finite number of pixels, it is only possible to obtain a limited number of gray shades. For example, where thirteen pixels are in a cell only thirteen shades of gray are possible. This is because each of the 13 pixels must be either on or off.
Adaptive dither is another technique that can be used in a screening module. With this technique, gray shades are simulated using very small dots. Lighter shades are represented with fewer dots than darker shades. An early example of this technique is given in U.S. Pat. No. 1,790,722 xe2x80x9cDuplex photo modulator.xe2x80x9d Other popular examples are shown in Floyd, R. W., and L. Steinberg, xe2x80x9cAn Adaptive Algorithm For Spatial Grayscalexe2x80x9d, Proc. SID, vol. 17/2, pp. 75-77 and Ulichney, R, xe2x80x9cDigital Halftoningxe2x80x9d, pp. 279-283. Adaptive techniques seek to cause an error signal representing the difference between the screened output and the input to approach zero. Typically, adaptive techniques generate a screened image exhibiting a large number of very small uniform size dots
While the adaptive technique often provides better detail reproduction, less objectionable patterns, and a larger number of distinguishable shades of gray than the conventional screening technique, it has the further disadvantage of generating screen patterns that are too fine to be well reproduced by most marking devices. This problem is especially severe in reproducing gray shades in the 50% gray region.
As discussed above nothing in the state of the art suggests varying the size of dots generated by an adaptive technique.
In view of the limitations of the above related art, it is an object of the invention to provide a photographic image reproduction device wherein photographic images are reproduced with good reproduction of detail. Another object of the invention is to provide a photographic reproduction device wherein a large number of gray shades can be distinguished.
Yet another object is to provide a photographic reproduction device wherein the resulting screening patterns appear pleasing to the eye.
A further object of the invention is to provide a photographic reproduction device wherein the resulting screening patterns can be reproduced accurately by common types of marking devices.
It is still another object of the invention to vary the size of dots generated by an adaptive technique in photographic reproduction.
The above objects of the invention are accomplished by a screening method and apparatus which allows the use of recursive or adaptive techniques to vary the size of dots used in producing the image. It should be noted that the dots can be produced by marking (e.g., black) and not marking (e.g., white) the medium storing the image produced. Any printing, electronic, or other storage medium which can represent an image by stored values is within the scope of the invention. In addition, although black and white dots are contemplated, other color dots can also be used within the scope of the invention. The resulting irregular placement of the dots significantly improves the number of gray shades which can be produced by the marking device. The recursive technique operates in two dimensions and employs a xe2x80x9chysteresisxe2x80x9d or adjustable coarseness constant which is used with an error signal to determine the coarseness of the image.
The photographic reproduction device according to the invention has a random access memory (RAM), a scanning device, a screening device which uses the RAM for input, output, and temporary storage of intermediate results, and a marking device. The screening device cycles through three phases of operation. In the first phase, image data from the scanning device is stored in an input area of the RAM. During the second phase, the screening patterns are computed, and stored in an output area of the RAM. During the third phase, the resulting screening patterns stored in the output area of the RAM are output to the marking device. Each of these phases of operation comprises an iteration through the same number of data elements as there are pixels in one scan line. During alternate screening processing phases, the scanning proceeds left-to-right, then right-to-left, to avoid objectionable diagonal line patterns.
It would be known to one of ordinary skill in the art that processing for the screening device can be implemented using a software program running on a general or special purpose computer or can be implemented using special purpose electronic circuitry. Both such implementations would be within the scope of the invention.